The present invention relates to a ball bushing for supporting a rod for carrying out a linear motion or a rotational motion.
In general, in a ball bushing of this type, a cylindrical retainer is fitted into a cylindrical outer tube and a plurality of balls are respectively retained for rolling in a plurality of annular grooves formed in the retainer. The ball bushing is press-fitted into a bearing hole of a machine having a rod for carrying out a linear motion or a rotational motion and the rod is inserted into the retainer and guided by the balls, thereby allowing a smooth motion of the rod.
However, in the above prior-art ball bushing, press-fitting resistance may be too large to smoothly position the outer tube in the hole portion in some cases in press-fitting the outer tube into the bearing hole in the machine, which degrades operability or causes fitting in of the ball bushing in an inclined state. As a result, reduction in yield is likely to be cased. Particularly, if a whole outer peripheral face of the outer tube is formed into a circumferential face with a uniform diameter, large friction force is generated by total surface contact of the outer peripheral face of the outer tube and an inner peripheral face of the bearing hole in press-fitting the outer tube into the bearing hole. As a result, not only large press-fitting force is required for fitting in of the ball bushing, but also the outer tube is deformed inward by large stress that acts on the outer tube to hinder smooth rolling of the balls.
The present invention has been accomplished with the above problems in view and it is an object of the invention to provide a ball bushing which can reduce press-fitting force and an amount of deformation in fitting the ball bushing into a bearing hole in a machine and which can be easily and reliably fitted in.
To achieve the above object, according to the invention there is provided a ball bushing comprising a cylindrical retainer into which a rod for carrying out a linear motion or a rotational motion can be inserted, a plurality of balls respectively housed for rolling in a plurality of annular grooves cut in the retainer and in rolling contact with the rod, and an outer tube fitted on an outer face of the retainer for retaining the balls from outside, the ball bushing being used in a press-fitted state in a bearing hole of a machine having the rod.
The outer tube has tapered portions inclined such that diameters of the tapered portions reduce as the tapered portions extend outward respectively at axial opposite end portions of an outer peripheral face, fixed portions having uniform outer diameters and pressed against a hole face of the bearing hole respectively in positions adjacent to the respective tapered portions on insides of the tapered portions, and a smaller-diameter portion having an outer diameter smaller than that of the fixed portions at at least a portion of a tube intermediate portion positioned between the two fixed portions.
According to the ball bushing of the invention having the above structure, by forming the tapered portions at the opposite end portions of the outer tube, the ball bushing can be easily press-fitted into the bearing hole of the machine without inclination in a state in which axes are aligned with each other by using guide functions of the tapered portions. By forming the fixed portions pressed against the hole face of the-bearing hole at the opposite end portions of the outer tube and forming the smaller-diameter portion at the tube intermediate portion, contact area of the outer tube and the bearing hole can be reduced to reduce frictional resistance in press-fitting, thereby facilitating the press-fitting. Stress that acts on the outer tube is also reduced to suppress deformation of the outer tube, thereby ensuring smooth rolling of the balls.
According to a preferred concrete embodiment of the invention, the balls are disposed in a line throughout an area extending from one of the fixed portions to the other, the ball line inclines in such a direction as to gradually separate from the rod in positions corresponding to both the fixed portions, the respective balls are in rolling contact with the rod in the tube intermediate portion, the whole tube intermediate portion is formed as the smaller-diameter portion, and inner tapered portions respectively inclined in reverse directions to the tapered portions at the opposite ends of the outer tube are formed between the smaller-diameter portion and both the fixed portions.
The tapered portions at the opposite ends of the outer tube and the inner tapered portions have the same inclination angles.
In the invention, it is possible that the ball bushing is partitioned into a plurality of areas in an axial direction and that the plurality of annular grooves for respectively retaining the balls are formed in each the area.